1 Field of the Invention
The invention relates to improvements in substrates and in particular to new substrates having magnetic and visual security features, which provide security against imitation.
2 The Prior Art
It is widely known to use in banknotes and other security documents security elements, such as security threads or strips. These threads are partially or wholly embedded in a paper or plastic substrate, and generally provide different viewing conditions depending on whether the security document is viewed in transmitted or reflected light.
EP-A-319157, for example, describes a security element made from a transparent plastic film provided with a continuous reflective metal layer, such as aluminium, which has been vacuumed deposited on the film. The metal layer is partially demetallised to provide clear demetallised regions which form indicia. When wholly embedded within a paper substrate the security element is barely visible in reflected light. However, when viewed in transmitted light the indicia can be clearly seen highlighted against the dark background of the metallised area of the thread and adjacent areas of the paper. Such threads can also be used in a security document provided with repeating windows in at least one surface of the paper substrate at which the security thread is exposed. A security document of this type, when viewed in transmitted light, will be seen as a dark line with the indicia highlighted. When viewed in reflected light on the windowed side, the bright shiny aluminium portions are readily visible in the windows. This thread has been highly successful within the market place and is supplied under the trade mark Cleartext®.
For a number of years banknote issuing authorities have had an interest in combining both the public recognition properties of Cleartext® with the covert properties of a machine readable feature. To this end it is preferable to utilise machine readable features that can be read using detectors already available to the banknote issuing authorities. Examples of such machine readable devices are described in WO-A-92/11142 and EP-A-773872.
The security device of WO-A-92/11142 is an attempt to provide this combination. A security device conforming to this specification has been used commercially with some success. A central region of the security device has a metallic appearance with clear regions forming characters; on either side of this central strip in the width direction, there are layers of magnetic material with obscuring coatings to provide the necessary magnetic component. This is, however, a generally unsatisfactory means of achieving the combination of the appearance of Cleartext® with the required magnetic properties. The magnetic properties are satisfactory, but the requirement to place the magnetic layers on either side of a central region means that the latter must be relatively narrow with respect to the overall thread width and results in characters which are small, typically 0.7 mm high, and therefore not easily legible. Additionally, the structures of the devices described in WO-A-92/11142 are very complex and present substantial lateral registration problems in depositing the various layers; a misregistration of even 0.1 mm or so can allow the presence of the dark magnetic oxide to be apparent to the naked eye, thus revealing its presence and seriously detracting from the aesthetic appearance of the security thread.
A more satisfactory solution, from the processibility, ease of character recognition and aesthetics points of view, would be to manufacture a device of the kind described in EP-A-0319157 from a metal which is itself magnetic such that the size of the characters and ratio of character height: thread width of the Cleartext® product is maintained, whilst providing direct compatibility with existing magnetic thread detectors. One means of achieving this is disclosed in Research Disclosure No. 323054 of March 1991. In this Research Disclosure, a magnetic material is deposited onto a flexible substrate by vacuum sputtering or other known techniques; the non-metallised regions are created by selective printing of a resist layer and subsequent chemical etching. The disclosed magnetic materials may be nickel, cobalt, iron or alloys thereof with a preferred combination of cobalt:nickel in the ratio 85:15%. The disadvantage of this method is that vacuum deposition of cobalt: nickel to the necessary thickness is a relatively slow process and somewhat wasteful of cobalt, an expensive material. Furthermore, subsequent to this vacuum deposition process, further significant processing is required to etch the characters. The resultant product is therefore relatively expensive.
A further alternative approach is described in EP-A-773872 wherein a magnetic metal is deposited on a film of polymeric substrate as the substrate passes through a solution containing the magnetic metal, and a preparatory operation is carried out on a surface of the substrate prior to immersion of the substrate in the solution. The preparatory operation ensures that magnetic metal is deposited on the substrate in a chosen pattern such that when the security product is produced from the film by cutting the film, the magnetic metal on the security thread has a specific pattern and provides both a visual discernible security feature and a magnetically detectable security feature. This method produces a security thread with satisfactory visual and machine readable characteristics but the manufacture is not straight forward and is costly.
One further approach is detailed in WO-A-9928852. Here the security device includes a carrier substrate, a metallic layer disposed on the carrier substrate, and a magnetic layer disposed on the metallic layer in substantial registration with at least a portion of the metallic layer, thereby providing both metallic security features and magnetic security features. The metallic layer and the magnetic layer also form graphic or visually identifiable indicia on the carrier substrate to provide a visual security feature. According to one method, the metallic layer is applied to the carrier substrate, the magnetic layer is applied to the metallic layer, and the layers are etched to form the graphic indicia. The magnetic layer can, in one embodiment, include a magnetic chemical resist that is printed on the metallic layer in the form of the graphic indicia. This method again produces a security device with acceptable visual and magnetic characteristics but again has a high cost with regard to processing and production.
The present invention therefore seeks to provide a security substrate that may be slit into security threads for partially or wholly embedding into paper or polymer which has acceptable magnetic and visual characteristics as described above and also greatly simplifies the manufacturing process. Such a simplification produces costs savings for both manufacture and materials as levels of spoil are greatly reduced.
A security substrate comprising a transparent polymer carrier layer bearing indicia formed from a plurality of opaque and non-opaque regions and a transparent magnetic layer supported by the carrier layer containing a distribution of particles of a hard magnetic material of a size and distributed in a concentration at which the magnetic layer remains transparent.
A preferred embodiment of the present invention will now be described by way of example only, with reference to the accompanying drawings.